The start of legal marijuana sales in Colorado may have reversed a rising trend of prescription opioid overdose deaths in the state, a new study set to be published next month concludes.

The study found that — even after taking into account other factors — nearly one fewer person per month died of an opioid overdose in Colorado after the start of legal cannabis sales in 2014 compared to before. The paper’s authors stop short of saying that legalization caused the reversal, instead saying that legalization was “associated” with a decline in opioid deaths. The authors also caution that the study looks only at a small sliver in time because legalization is still relatively new.

“These initial results clearly show that continuing research is warranted as data become available, involving longer follow-ups and additional states that have legalized recreational cannabis,” the study’s authors write.

Dr. Larry Wolk, the executive director of the Colorado Department of Public Health and Environment, said it is too soon to draw sweeping conclusions about legalization’s impact.

“It just hasn’t been in place long enough,” Wolk said. “Anything that does get published at this point should be considered preliminary data.”

Robert Valuck, who coordinates the Colorado Consortium for Prescription Drug Abuse Prevention, said there are too many factors at work in the state’s opioid death trends to isolate the impact of one policy change.

“The whole thing is so convoluted, with so many different things going on in the marketplace, it’s virtually impossible to assign cause and effect or credit and blame to any one thing,” he said.

The paper, which was peer-reviewed, is available online now and is slated for the November issue of the American Journal of Public Health. Its authors work at the University of North Texas Health Science Center, as well as at the University of Florida and Emery University in Atlanta.

The researchers charted deaths from prescription opioid painkillers in Colorado from the start of 2000 through the end of 2015. They compared those numbers to data from Nevada and Utah — to account for trends that cut across state lines. They also attempted to account for the impact of changes in 2014 to strengthen Colorado’s prescription drug monitoring program, which aims to reduce prescription drug addiction.

Throwing all of this into a statistical model, the researchers found a 6.5 percent decrease in monthly opioid deaths after legal recreational marijuana sales began in 2014.

To negate the study’s conclusion about marijuana’s role, the authors write, any alternate explanations for the decline would have to have occurred around the time legal sales started and have had a disproportionate impact in Colorado compared to neighboring states.

Other studies have also suggested liberalized marijuana laws could reduce prescription drug abuse.

A study in 2014 concluded that state medical marijuana laws were associated with lower opioid-overdose death rates. Other studies have found that marijuana is effective at controlling chronic pain — meaning people may choose it over opioids when they have the legal option — and that painkiller prescriptions drop after states adopt medical marijuana laws.

But Wolk and Valuck said the correlation is too simple.

In addition to the stronger monitoring program, 2014 also saw increased public education about the dangers of opioid prescribing and wider distribution of the overdose-reversing drug naloxone. The Colorado Consortium for Prescription Drug Abuse Prevention, which brings doctors, pharmacists, policy officials and others together, was also just getting up to speed.

Valuck also noted that, even as prescription opioid deaths have fallen, deaths from heroin have been rising in Colorado — meaning it is possible that what appears to be progress in combating prescription drug addiction is actually just a large-scale switch to a different opiate.

Valuck said researchers struggle to quantify the impact of any one of those factors, making it unlikely any researcher could separate out the impacts of all of them from another potential factor. But, for the same reason, he said researchers can’t be certain that marijuana legalization didn’t have an impact.

“Everybody wants the answer now because we want to know if this is a good idea or not,” Valuck said. “But the truth is we don’t have the answer, and it’s going to be a while until the jury comes back in.”

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